Atomized steel powder having improved hardenability

ABSTRACT

Atomized steel powder having improved hardenability by the addition of boron. The powder is produced by atomizing a molten stream of steel containing 0.06 to 0.12 percent carbon and 0.005 to 0.100 percent boron. Following the atomization, the particles are annealed at a temperature of about 1500* to 2100* F. to soften the steel, as well as reducing the carbon content to a value below 0.05 percent. The annealed steel contains about 0.001 to 0.012 percent boron. After annealing, the cake-like structure is broken up to restore the as-atomized particle size and the particles are subsequently compacted into the shape of the desired part, sintered and heat treated to develop the desired hardness. The addition of boron aids in improving the hardenability of the part during heat treatment.

llnited States Patent [191 Huseby [54] ATOMIZED STEEL POWDER HAVINGIMPROVED HARDENABILITY Robert A. Huseby, Milwaukee, Wis.

[73] Assignee: A. 0. Smith-Inland Inc., Milwaukee,

Wis.

[22] Filed: Aug. 23, 1971 [21] Appl. No.: 174,159

[75] Inventor:

[52] US. Cl. ..l48/16, 75/.5 BA, 75/202, 75/211,148/16,148/126, 29/182,29/182.5 [51] Int. Cl. ..B22f 3/24, B22f 1/00 [58] Field of Search..75/202, 200, 201, 123, 128, 75/211;29/182, 182.5; 148/16, 31, 36,126

[45] Apr. 3, 1973 3,528,081 9/1970 Huseby et a1 ..75/.5 BA

Primary Examiner-Charles N. Lovell AttorneyAndrus, Sceales, Starke &Sawall [57] ABSTRACT Atomized steel powder having improved hardenabilityby the addition of boron. The powder is produced by atomizing a moltenstream of steel containing 0.06 to 0.12 percent carbon and 0.005 to0.100 percent boron. Following the atomization, the particles areannealed at a temperature of about l500 to 2100 F. to soften the steel,as well as reducing the carbon content to a value below 0.05 percent.The annealed steel contains about 0.001 to 0.012 percent boron. Afterannealing, the cake-like structure is broken up to restore theas-atomized particle size and the particles are subsequently compactedinto the shape of the desired part, sintered and heat treated to developthe desired hardness. The addition of boron aids in improving thehardenability of the part during heat treatment.

11 Claims, No Drawings ATOMIZED STEEL POWDER HAVING IMPROVEDHARDENABILITY BACKGROUND OF THE INVENTION Steel powder to be used inpowder metallurgy processes can be prepared by a number of differentprocedures, such as electrolytic processes, reduction processes, or byair or by water atomization processes as described in the U.S. Pat. No.3,325,277 of Robert A. Huseby. According to the process of that patent,

molten steel is fed by gravity in the form of a downwardly moving streamand a series of flat sheets of water are impinged against the stream ofmolten steel at an angle to thereby atomize the stream and produce aplurality of agglomerates of spheroidal steel particles. Subsequently,the particles are annealed in a reducing atmosphere for a period of timesufficient to soften the particles and reduce the carbon content.Following the annealing, the particles are subjected to hammermilling tobreak up the cake-like structure formed during the anneal and restorethe as-atomized particle size.

The steel powder formed according to the method of the aforementionedpatent after annealing has a low carbon content, generally less than0.05 percent and a relative high oxygen content, up to 0.40 percent.

In the past, atomized steel powder was normally used to fabricate smallparts of relatively light section and hardenability was not a problem.More recently, steel powder has been used to form larger parts of heavysection, and with the heavier sections, hardenability is increasinglyimportant. Hardness is determined by heat treatment and hardenability isthe ability for the hardness to penetrate through the section. Steelpowder formed by an atomization process is extremely fine grained, andmetallurgically, any fine grained structure has inherently poorhardenability.

It is recognized that the addition of boron to steel will improve thehardenability of the steel. It is also known that boron has deoxidizingand denitrifying characteristics in themelt, and because of this, theattainment of a substantial boron content in the steel is difficult,particularly where a specific boron content is desired. To prevent theloss of boron in the melt through deoxidizing and denitrifying actions,steel has generally been degasified before the addition of boron, asdescribed in U.S. Pat. No. 2,823,299, by the addition of deoxidizingagents, such as aluminum, silicon and manganese. These metals react withthe oxygen and nitrogen to remove the same so that the loss of boron inthe melt will be minimized.

While the addition of deoxidizing agents such as aluminum, silicon andmanganese to the melt has been successful in minimizing the loss ofboron, this procedure has not been utilized in steel powder atomizationprocesses, for the aluminum killed steel will freeze the tundish so thatthe molten steel will not properly flow through the outlets for theatomization, and theaddition of silicon and manganese substantiallyreduces the mechanical properties of the resulting compacted powder. Inview of this, and the fact that the steel to be atomized is alow-carbon, high-oxygen type, boron has not been considered a prospectfor increasing the hardenability of steel powder.

SUMMARY OF THE INVENTION The invention relates to an atomized steelpowder which has increased hardenability due to the addition of boron.According to the invention, steel powder is produced by atomizing amolten stream of steel containing 0.06 to 0.12 percent carbon and 0.005to 0.015 percent boron. Following the atomization, the resultingparticles are annealed at a temperature of about 1500 to 2100 F tosoften the steel and reduce the carbon content to a value in the rangeof about 0.01 to 0.05 percent. The annealed steel contains about 0.001to 0.002 percent boron.

After annealing, the cake-like structure is broken up by hammer-millingto restore the as-atomized particle size, and the annealed particles aresubsequently compacted into the desired shape of the part to be formedand sintered at a temperature in the range of 2000 to 2300 F. Followingthe sintering, the part is heat treated to develop the hardness byheating to a temperature in the range of about 1475 to 1650 F.,quenching and subsequently tempering at a temperature in the range of300 to 1000 F.

The addition of boron increases the hardenability of the sintered part,yet retains the fine grain structure of the particles, therebyincreasing the mechanical properties of the heat treated part.

DESCRIPTION OF THE PREFERRED EMBODIMENT The steel to be used in theprocess of the invention can be produced by one of the conventionalsteel making processes such as open-hearth, electric furnace, basicoxygen or the like. The steel contains from .001 to 0.20 percent carbon,generally in the range of 0.06 to 0.12 percent carbon and preferably inthe range of 0.03 to 0.08 percent carbon. In addition, the steel cancontain one or more of the following elements: 0.20 to 3.0 percentnickel, 0.20 to 1.0 percent chromium and 0.20 to 1.0 percent molybdenum.

The silicon and manganese should be maintained below certain limits. Thesilicon content of the steel should be maintained less than 0.10 percentby weight and in the range of 0.01 to 0.10 percent, while the manganesecontent should be less than 0.30 percent by weight and in the range of0.05 to 0.30 percent, but can be as high as 0.80 percent by weight whenthe alloy contains substantial additions of chromium, nickel and/ormolybdenum.

The titanium content of the alloy should be less than 0.05 percent byweight, the sulfur and phosphorus should be less than 0.04 percent and0.035 percent, respectively, and'the aluminum content should be lessthan 0.010 percent and preferably less than 0.005 percent.

According to the invention, boron is added to the melt in an amount of0.005 to 0.100 percent by weight and preferably in the range of 0.0075to 0.0500 percent by weight. The boron is preferably added in the formof The steel in the melt has a relatively low carbon content andinherently has a relatively high percentage of oxygen. If boron wasadded to the low-carbon, high-oxygen melt, one would normally expect theboron to be completely lost due to its deoxidizing and denitrifyingcharacteristics. As previously mentioned, the conventional procedure inthe past has been to substantially completely kill the steel prior tothe addition of boron by use of a deoxidizing agent, such as aluminum,silicon or manganese. While small amounts of aluminum, up to 0.010percent by weight, can be included in the steel without adverse effect,completely killing the steel with aluminum cannot be tolerated where thesteel is to be utilized in an atomization process because the aluminumkilled steel will be sluggish and will tend to freeze the tundish sothat the molten steel will not adequately flow through the outlet slotsfor the atomization procedure. Similarly, killing the steel with siliconand manganese cannot be tolerated because silicon and manganese, duringatomization and the subsequent annealing treatments, form oxides whichare extremely refractive and are difficult to reduce during the anneal.This results in the powder having a high oxide content in the form ofoxide inclusions which reduces the ductility, impact strength andfatigue strength of the resulting compacted powder.

According to the process of the invention, about 5 to 25 percent byweight of the melt, prior to the addition of boron, is initially killedwith aluminum to substantially remove all oxygen and nitrogen from thatportion of the melt. The entire quantity of the boron to be included inthe alloy is then added to the killed portion of steel, and aftersolution of the boron, the remaining unkilled portion of the melt isadded. As the boron is in solution in the small portion of the killedsteel, reaction of the boron with the oxygen or nitrogen in the unkilledportion of the steel will be minimized, with the result that asubstantial portion of the boron will be retained in the melt. The smallamount of aluminum required to kill the minor portion of the melt willnot adversely affect the characteristics of the melt in the tundish sothat the molten steel will satisfactorily flow from the tundish in theform of molten streams.

Alternately, the steel in the melt can be substantially completelykilled prior to the addition of the boron by use of a calcium alloy,such as calcium-silicon, which contains approximately 30 to 33 percentby weight of calcium, 60 to 65 percent silicon and 1.5 to 3 percentiron, or calcium-manganese-silicon which contains 16 to percent ofcalcium, 14 to 18 percent manganese and 54 to 59 percent silicon. Anyexcess calcium, as well as the resulting oxides, will go off in theslag. Calcium is a strong deoxidizer, and as only a very small amount ofthe calcium alloy is required for killing the steel, the silicon ormanganese present in the calcium alloy will not significantly contributeto the overall silicon or manganese content of the melt, so that themechanical properties of the resulting compacted powder will not beseriously decreased. In some cases, small amounts of aluminum, below themaximum content set forth previously, can be incorporated with thecalcium alloy.

The steel powder can be produced by an apparatus similar to that shownin U.S. Pat. No. 3,325,277. The molten steel is contained in a tundishat a temperature of about 3100 F. and flows by gravity from the tundishthrough a series of outlet slots or nozzles. A thin sheet or curtain ofwater is directed against the stream of molten steel at an angle greaterthan 5 with respect to the axis of the stream and generally at an angleof 15 to 55 from the vertical.

The temperature of the water employed in the atomization process is notcritical and is generally less than 160 F. The water is undersubstantial pressure, usually above 500 psi and for most operations,above 1000 psi. There is no maximum pressure limit for the water andnormally the maximum pressure is based on the pumping equipment used. Inthe atomization, the water pressure is correlated to the angle at whichthe water sheets are directed against the molten metal stream. As theangle is decreased and approaches the vertical, the water pressure mustcorrespondingly increase. Generally, the horizontal component of watervelocity should be above 105 feet per second to produce the desiredagglomerated type of particles.

The water is preferably in the form of thin sheets having a thicknessless than 0.075 inch and preferably less than 0.05 inch at the point ofdischarge from the nozzle. The nozzles are designed with respect to themolten streams so that the sheets of water do not flair out to anyappreciable extent but maintain the thickness when impinging against themolten steel stream.

The thin sheets of water strike the molten steel stream and atomize orparticalize the steel to produce chain-like agglomerates of generallyspheroidal particles. The steel powder as-atomized has a particle sizesuch that at least percent will pass through an 80 mesh sieve and atleast 75 percent will pass through a mesh sieve.

Following the atomization, the steel powder is subjected to an annealingtreatment which serves to soften the particles, reduce the oxide filmand substantially decrease the carbon content. During the anneal, thepowder is heated to a temperature in the range of 1500 to 2100 F. andpreferably 1650 to 1850" F. in a reducing atmosphere such asdisassociated ammonia, hydrogen or other conventional decarburizingreducing gases.

During the annealing, the particles are softened, the carbon content isreduced and the oxygen content is reduced. The annealed particles have acarbon content below 0.05 percent by weight and preferably in the rangeof 0.001 percent to 0.020 percent. The annealed powder has an oxygencontent less than 0.40 percent by weight and in most cases in the rangeof 0.01 to 0.25 percent. If chromium is not used in the alloy steel, orif the chromium content is in the lower portion of its aforementionedrange, the oxygen content will generally be below 0.25 percent. If thechromium content is in the upper portion of its aforementioned range,the oxygen content may be above 0.25 percent but below 0.40 percent.

To obtain the optimum ductility and. subsequently obtain the maximumdensity for a given compaction pressure and increased physicalproperties in the sintered product, the powder should be maintained atthe annealing temperature for a period of at least l )5 hours andpreferably about 2 hours.

As previously mentioned, a portion of the boron added to the melt islost, but the annealed powder will generally have a boron content in therange of 0.001 to 0.012 percent and preferably in the range of 0.002 to0.005 percent.

Following the anneal, the particles are generally caked together and arebroken apart by hammermilling process. The hammer-milling which is animpact process breaks the sintered cake while not breaking up theirregular agglomerated nature of the particles and serves to restore theas-atomized particle size.

The annealed powder has an apparent density, which is a non-compacteddensity as defined by test procedure ASTMB-212-48, in the range of 2.6t0'3.3 grams/cc. The steel powder has a pressed density of over 6.4grams/cc and generally in the range of 6.4 to 6.8 grams/cc. The presseddensity is based on a compaction pressure of 30 tons per square inch, asdefined in the test procedure ASTMB-33l-5 8T, except that 0.5 percentdry zinc stearate lubricant was mixed with the powder.

The steel powder can be used to form various parts or combination ofparts of both light and heavy section by conventional powder metallurgyprocedures. A conventional lubricant such as zinc stearate andadditional carbon, if desired, can be blended with the steel powder bysuitable blending equipment. The powder is then compacted into thedesired shape by a compaction pressure generally above tons per squareinch and preferably about 30 tons per square inch or more.

Following the compaction, the steel powder is sintered in a reducingatmosphere at a temperature in the range of 2000 F. to 2300 F. for aperiod of 10 minutes to 1 hour, depending on the composition and thefinal density desired. 7

After the sintering, the sintered part is subjected to a heat treatmentto develop hardness throughout the section thickness and improve itsphysical properties. In

the heat treatment, the part is initially heated to a temperature in therange of about l475to 1850" F. for a period of time sufficient to permitthe entire depth of the section to be at temperature. The part is thenquenched, either by water or oil, and subsequently tempered at atemperature of 300 to 1000 F. The quench develops the hardness in thepart, while the tempering improves the elongation and impact strength.

The heat treated part can have a hardness up to about 65 Rickwell-C andthe specific hardness is determined primarily by the carbon content ofthe steel. The addition of boron does not, in itself, increase thehardness of the part, but improves the hardenability which is theability to harden throughout the entire section. The boron has theability to increase the hardenability at low cost, as compared to otheralloying elements.

The addition of boron to the steel powder provides a substantialincrease in hardenability for the steel powder while maintaining thefine grained structure which is necessaryfor optimum mechanicalproperties. The boron addition is obtained through the invention withoutsubstantiallycompletely killing the steel in the melt with aluminum andwithout any increase in the content of silicon and manganese which aredetrimental to the properties of the steel powder.

The following examples illustrate the process of preparing the steelpowder of the invention: Rockwelllowing composition in weight per centwas supplied to the ladle:

Carbon 0.100 Manganese 0.150 Phosphorus 0.010 Sulfur 0.018 Silicon 0.022lron balance Aluminum powder was added to the ladle in sufficientquantity to substantially kill the steel. Ferroboron containingapproximately 18 percent by weight of boron was then added to the ladlein an amount such that elemental boron composed 0.0050 percent of thetotal melt. After solution of the ferro-boron, the remaining 90 percentof the melt was added to the ladle.

The molten steel was then supplied to a tundish and with the temperatureof the steel at approximately 3100 F., the steel flowed downwardly bygravity throughout outlet nozzles having an internal diameter ofseven-sixteenths inch. Two oppositely directed streams or curtains ofwater positioned at a downward angle of 33 with respect to the axis ofthe molten stel streams impinged against the streams to atomize thesteel. The temperature of the water was initially 68 F. and had a finaltemperature of 138 F. The water was under pressure of 1050 psi and at aflow rate of 860 gallons per minute. The water streams were dischargedthrough slots 3 inches long and 0.04 inches wide.

The resulting as-atomized steel powder had the following Tyler screenanalysis:

Tyler Screen Analysis +100 +150 +200 +250 +325 trace 2.6 16.8 26.8 5.723.3 24.8

The steel powder was then annealed in dissociated ammonia at atemperature of 1,700 F. for 2 hours, subsequently cooled in adissociated ammonia atmosphere to 140 F. and then air cooled to toomtemperature.

The annealed steel powder had the following analysis in weight per cent:

Carbon 0.014 Manganese 0.150 Phosphorus 0.010 Sulfur 0 .0 1 2 Silicon0.022 ()xygen 0.1 88 Boron 0.0016 lron balance The steel powder was thenbroken up by hammermilling to as-atomized size arid the powder had anapparent density of 3.08 grams per cc, a green density at a compactionpressure of 30 tons per square inch with 0.75 percent zinc stearatelubricant of 6.70 grams per cc and a green strength of 1500 per squareinch with 0.75 percent zinc stearate lubricant after pressing at 30 tonsper square inch.

A sample of the annealed steel powder was compacted into the shape of atest bar having dimensions of 0.25 X 0.50 X 1.25 inches at a pressure of32 tons per square inch. The compressed part was then sintered at atemperature of 2050 F. for a period of )6 hour. Subsequently, thesintered part was heated to a temperature of 1650 F. for a period ofone-half hour, quenched in oil and tempered at a temperature of 600 F.for 1 hour.

The resulting heat treated part had a hardness of 26 Rockwell-C whichpenetrated throughout the section thickness.

EXAMPLE Il Ten lbs. of aluminum and 18 lbs. of calcium silicon wereadded to 48,000 lbs. of molten alloy steel in the ladle and boron in theform of ferro-boron was added to the steel in the ladle in an amountsuch that the melt contained 0.0075 percent of elemental boron.

The final tap analysis of the steel was as follows in weight per cent:

Carbon 0.05 Manganese 0.37 Molybdenum 0.61 Nickel 0.5 l Boron 0.0010Silicon 0.012 lron balance The molten steel was then atomized, annealedand hammer-milled according to the procedure outlined in Example I. Theannealed steel powder had the following analysis in weight per cent:

Carbon 0.01 Manganese 0.35 Nickel 0.5 l Molybdenum 0.61 Silicon 0.02Boron 0.0010 Oxygen 0.1 5 lron balance The annealed steel powder had anapparent density of 2.89 gr/cc, a green density at a compaction pressureof 30 tsi and with 0.75 percent zinc stearate lubricant of 6.74 gr/ccand a green strength of 1189 psi with a 0.75 percent addition of zincstearate lubricant after pressing at 30 tsi.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

1 claim:

1. Finely divided annealed steel powder having improved hardenabilityand to be used in powder metallurgy processes, comprising a plurality ofagglomerated steel particles, said steel consisting essentially of .001to 020% by weight of carbon, 0.001 to 0.012 percent by weight of boron,less than 0.8 percent manganese, less than 0.1 percent silicon, lessthan 0.01 percent aluminum, and the balance iron said steel particleshaving improved hardenability when subsequently heat treated aftercompaction and sintering.

2. Finely divided annealed steel powder having improved hardenability tobe used in powder metallurgy processes, comprising a plurality ofagglomerated steel particles consisting essentially by weight of 0.001to 0.020 percent carbon; an element selected from the group consistingof 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0percent molybdenum and mixtures thereof; 0.01 to 0.80 percent manganese;0.01 to 0.10 percent silicon; 0.001 to 0.002

percent boron; and the balance iron; said particles having improvedhardenability on heat treatment after compaction and sintering.

3. The steel powder of claim 2, wherein said annealed steel has anoxygen content less than 0.40 percent by weight.

4. A method of forming a steel part from a plurality of agglomeratedsteel particles, comprising the steps of atomizing a stream of moltensteel consisting essentially of 0.06 to 0.12 percent by weight ofcarbon, and 0.005 to 0.100 percent by weight of boron, less than 0.8percent manganese, less than 0.1 percent Si, and the balance iron tothereby provide a plurality of agglomerated steel particles, annealingthe agglomerated particles in a reducing atmosphere at a temperature ofl500 to 2100" F. for a period of time to soften the particles and reducethe carbon content to a value less than 0.05 percent by weight andreduce the oxygen content to a value less than 0.40 percent by weight,compacting the annealed particles into the desired shape of a part,sintering the part at a temperature in the range of 2000" to 2300 F.,heating the sintered part to a temperature in the range of 147 5" to1850 F., quenching the part, and tempering the part at a temperature inthe range of 300 to 1000 F., the addition of boron improving thehardenability of the steel during the heat treatment following thecompaction and sintering.

5. The method of claim 4, wherein the annealed particles have a boroncontent in the range of 0.001 to 0.012 percent by weight.

6. The method of claim 4, wherein the steel consists essentially byweight of 0.001 to 0.020 percent carbon; an element selected from thegroup consisting of 0.20 to 3.0 percent nickel, 0.20 to 1.0 percentchromium, 0.20 to 1.0 percent molybdenum and mixtures thereof; 0.01 to0.80 percent manganese; 0.01 to 0.10 percent silicon; 0.001 to 0.002percent boron; and the balance iron.

7. A method of forming a steel part from a plurality of agglomeratedsteel particles, comprising the steps of forming a steel melt consistingessentially of from 0.06 to 0.12 percent by weight of carbon, less than0.8 percent manganese, less than 0.1 percent silicon and the balanceiron, adding a deoxidizing agent to only a portion of the melt todeoxidize said portion, said portion comprising from 5 to 25 percent byweight of the melt, adding boron to the deoxidized portion of the meltin an amount of 0.005 to 0.100 percent by weight of the entire melt andeffecting solution of said boron in said portion, adding the remainingportion of the melt to said deoxidized portion to provide a blendedmelt, atomizing the blended melt to provide a plurality of agglomeratedsteel particles containing from 0.001 to 0.012 percent by weight ofboron, annealing the particles to soften the particles and reduce'thecarbon content to a value in the range of 0.001 to 0.050 percent byweight, compacting the annealed particles into the desired shape of apart, sintering the part, heating the sintered part to a temperature inthe range of 1475 to 1850 F., quenching the part, and tempering the partat a temperature in the range of 300 to 1000 F., the addition of boronimproving the hardenability of the steel during the heat treatmentfollowing the compaction and sintering.

8. The method of claim 7, in which the deoxidizing agent is aluminum andis added to said portion of the melt in an amount sufficient tosubstantially completely deoxidize said portion, said aluminumcomprising less than 0.010 percent by weight of the steel particles.

9. A method of forming a steel part from a plurality of agglomeratedsteel particles, comprising the steps of forming a steel melt consistingessentially of from 0.06 to 0.12 percent by weight of carbon, less than0.8 percent Mn, less than 0.1 percent silicon, and the balance iron,adding a calcium alloy to the melt in an amount sufficient to deoxidizethe melt, adding boron to the melt in an amount sufficient to provide anelemental boron content of 0.001 to 0.012 percent by weight of the melt,atomizing a stream of the melt to thereby provide a plurality ofagglomerated steel particles, annealing the agglomerated particles in areducing atmosphere at a temperature of 1500 F. for a period of time tosoften the particles and reduce the carbon con tent to a value less than0.05 percent by weight and reduce the oxygen content to a value lessthan 0.40 percent by weight, compacting the annealed particles into thedesired shape of a part, sintering the part, heating the sintered partto a temperature in the range of 1475" to 1850 F., quenching the part,and tempering the part at a temperature in the range of 300 to 1000 F.,the addition of boron improving the hardenability of the steel duringthe heat treatment following the compaction and sintering.

10. The method of claim 9, wherein the calcium alloy is selected fromthe group consisting of calcium-silicon, calcium-manganese-silicon, andmixtures thereof.

11. The method of claim 9, wherein the calcium alloy is selected fromthe group consisting by weight of (a) 60 to 65 percent silicon, 30 to 33percent calcium and 1.5 to 3 percent iron, (b) 16 to 20 percent calcium,14 to 18 percent manganese and 54 to 59 percent silicon, and (c)mixtures of (a) and (b).

2. Finely divided annealed steel powder having improved hardenability tobe used in powder metallurgy processes, comprising a plurality ofagglomerated steel particles consisting essentially by weight of 0.001to 0.020 percent carbon; an element selected from the group consistingof 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0percent molybdenum and mixtures thereof; 0.01 to 0.80 percent manganese;0.01 to 0.10 percent silicon; 0.001 to 0.002 percent boron; and thebalance iron; said particles having improved hardenability on heattreatment after compaction and sintering.
 3. The steel powder of claim2, wherein said annealed steel has an oxygen content less than 0.40percent by weight.
 4. A method of forming a steel part from a pluralityof agglomerated steel particles, comprising the steps of atomizing astream of molten steel consisting essentially of 0.06 to 0.12 percent byweight of carbon, and 0.005 to 0.100 percent by weight of boron, lessthan 0.8 percent manganese, less than 0.1 percent Si, and the balanceiron to thereby provide a plurality of agglomerated steel particles,annealing the agglomerated particles in a reducing atmosphere at atemperature of 1500* to 2100* F. for a period of time to soften theparticles and reduce the carbon content to a value less than 0.05percent by weight and reduce the oxygen content to a value less than0.40 percent by weight, compacting the annealed particles into thedesired shape of a part, sintering the part at a temperature in therange of 2000* to 2300* F., heating the sintered part to a temperaturein the range of 1475* to 1850* F., quenching the part, and tempering thepart at a temperature in the range of 300* to 1000* F., the addition ofboron improving the hardenability of the steel during the heat treatmentfollowing the compaction and sintering.
 5. The method of claim 4,wherein the annealed particles have a boron content in the range of0.001 to 0.012 percent by weight.
 6. The method of claim 4, wherein thesteel consists essentially by weight of 0.001 to 0.020 percent carbon;an element selected from the group consisting of 0.20 to 3.0 percentnickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0 percent molybdenum andmixtures thereof; 0.01 to 0.80 percent manganese; 0.01 to 0.10 percentsilicon; 0.001 to 0.002 percent boron; and the balance iron.
 7. A methodof forming a steel part from a plurality of agglomerated steelparticles, comprising the steps of forming a steel melt consistingessentially of from 0.06 to 0.12 percent by weight of carbon, less than0.8 percent manganese, less than 0.1 percent silicon and the balanceiron, adding a deoxidizing agent to only a portion of the melt todeoxidize said portion, said portion comprising from 5 to 25 percent byweight of the melt, adding boron to the deoxidized portion of the meltin an amount of 0.005 to 0.100 percent by weight of the entire melt andeffecting solution of said boron in said portion, adding the remainingportion of the melt to said deoxidized portion to provide a blendedmelt, atomizing the blended melt to provide a plurality of agglomeratedsteel particles containing from 0.001 to 0.012 percent by weight ofboron, annealing the particles to soften the particles and reduce thecarbon content to a value in the range of 0.001 to 0.050 percent byweight, compacting the annealed particles into the desired shaPe of apart, sintering the part, heating the sintered part to a temperature inthe range of 1475* to 1850* F., quenching the part, and tempering thepart at a temperature in the range of 300* to 1000* F., the addition ofboron improving the hardenability of the steel during the heat treatmentfollowing the compaction and sintering.
 8. The method of claim 7, inwhich the deoxidizing agent is aluminum and is added to said portion ofthe melt in an amount sufficient to substantially completely deoxidizesaid portion, said aluminum comprising less than 0.010 percent by weightof the steel particles.
 9. A method of forming a steel part from aplurality of agglomerated steel particles, comprising the steps offorming a steel melt consisting essentially of from 0.06 to 0.12 percentby weight of carbon, less than 0.8 percent Mn, less than 0.1 percentsilicon, and the balance iron, adding a calcium alloy to the melt in anamount sufficient to deoxidize the melt, adding boron to the melt in anamount sufficient to provide an elemental boron content of 0.001 to0.012 percent by weight of the melt, atomizing a stream of the melt tothereby provide a plurality of agglomerated steel particles, annealingthe agglomerated particles in a reducing atmosphere at a temperature of1500* F. for a period of time to soften the particles and reduce thecarbon content to a value less than 0.05 percent by weight and reducethe oxygen content to a value less than 0.40 percent by weight,compacting the annealed particles into the desired shape of a part,sintering the part, heating the sintered part to a temperature in therange of 1475* to 1850* F., quenching the part, and tempering the partat a temperature in the range of 300* to 1000* F., the addition of boronimproving the hardenability of the steel during the heat treatmentfollowing the compaction and sintering.
 10. The method of claim 9,wherein the calcium alloy is selected from the group consisting ofcalcium-silicon, calcium-manganese-silicon, and mixtures thereof. 11.The method of claim 9, wherein the calcium alloy is selected from thegroup consisting by weight of (a) 60 to 65 percent silicon, 30 to 33percent calcium and 1.5 to 3 percent iron, (b) 16 to 20 percent calcium,14 to 18 percent manganese and 54 to 59 percent silicon, and (c)mixtures of (a) and (b).